Here's what City of London banks are telling clients ahead of tonight's crucial Brexit vote

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The City of London is advising clients on tonight's Brexit vote.

• British politicians will vote on Prime Minister Theresa May's Brexit Withdrawal Agreement Tuesday, with City of London banks spelling out a variety of options to clients.
• Citi has advised clients not to trade the pound today because of the possibility of huge swings in its valuation.
• Barclays have additional staff working on trading desks in Singapore, New York and London ahead of the vote.

On Tuesday evening, British members of parliament (MPs) will vote on Theresa May's Brexit deal, with the prime minister widely expected to suffer a crushing, and possibly record-breaking, defeat in the House of Commons.

While the outcome of the vote seems fairly clear, what happens next is not, and financial markets are scrambling for any helpful information in the hours before the vote.

City of London lenders have been advising clients for months in the build up to the vote in order to steer them through the chaos. Citigroup's private banking arm, for instance, has told clients not to trade the pound because of expected volatility Tuesday.

"Over the next 24 hours what all we're going to find out is the degree to which May loses, how much she loses by," said David Bailin, global head of investments at Citi Private Bank, in Singapore. "That is not something that one actually trades on."

Similarly, Barclays has brought in additional staff to cope with demand for information from clients ahead of the vote. with numerous staff set to manage potential risks from the decision's fallout. FX and rates teams will be in earlier and stay later in Singapore, London, and New York on Tuesday.

A rejection of the Withdrawal Agreement, which sets the terms for the UK's exit and transition period away from the European Union, "wouldn't come as a shock to the markets," according to Emmanuel Cau, head of European equities at Barclays.

The pound has gained 1% against the dollar in the past month despite Brexit related uncertainties. The world's fourth most liquid currency market could tank 10% if an unexpected or undesirable Brexit outcome occurs, according to Citi.

While markets are thought to have priced in a defeat for the Prime Minister - perhaps one of the largest in parliamentary history - the pound and UK equities could rally if the vote does unexpectedly pass.

Reports suggest that the opposition Labour party will trigger a vote of no confidence in the government immediately after it is voted down, which could ultimately lead to an election.

A variety of City of London advisors have already advised clients that a rejection of May's deal could lead to a UK general election in 2019.

The UK is currently set to leave the EU on March 29. The pound is flat against the dollar as of 9.20 a.m in London (4.20 a.m EST).